It is unfortunately the case that the "burden of proof" necessarily lies
on those would would modify Common Lisp. In particular, it is clearly
part of the charter of the Common Lisp standards group that changes to
Common Lisp only be made after explicit consideration of the costs
involved.
There is a very good 40-page paper by Richard Gabriel and Kent Pitman
which goes into some details of all of the complex interactions that
one-namespace would have with a variety of other features of Common
Lisp, including macros, default dynamic binding for free references,
etc.
I recommend it as required background reading.